When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup, reduced vehicle stopping distance and a deterioration in the lateral stability of the vehicle.
Numerous wheel lock control systems have been proposed to prevent the wheels from locking up while being braked. Typically, these systems prevent the wheels from locking by releasing the applied brake pressure when an incipient wheel lockup condition is sensed. One criterion that is used to sense an incipient wheel lockup condition is excessive wheel deceleration. After release of the brake pressure, the wheel deceleration ceases and the wheels then accelerate toward vehicle speed. When the wheel speed has substantially recovered, brake pressure is reapplied. One criterion that is typically used to sense recovery is wheel acceleration falling below a specified low value. Reapplication of brake pressure results in the wheel again approaching lockup and the cycle is repeated. This form of wheel lock control system results in a rapid cycling of the brake pressure and the wheel slip value around the critical slip value to prevent wheel lockup. Ten Hertz is a typical cycling frequency. Brake pressure modulators of various types have been used to cycle and limit the brake pressure at the vehicle wheel brakes to accomplish this.
As opposed to the foregoing systems for preventing wheel lockup during braking, the invention disclosed and claimed in U.S. Pat. No. 4,653,815 in which we are co-inventors, is directed toward improvements used in a system which identifies the wheel brake pressure that results in the wheel slip being at the critical slip value and which produces the maximum braking force between the tire and the road surface. The brake pressure so identified is then applied to the wheel brake by using wheel brake pressure control means including an actuator and embodying the invention claimed herein so as to substantially continuously establish the critical slip value between the wheel and the road surface resulting in the maximum possible braking effort.
In general, the system in which the invention of U.S. Pat. No. 4,653,815 and the invention herein claimed are employed repeatedly calculates the braking force between the wheel and the road surface during braking from system constants and measured values and stores the brake pressure corresponding in time to the peak calculated force. When an incipient wheel lockup is detected indicating that the critical wheel slip value and therefore the peak braking force between the wheel and road surface has been exceeded, the stored brake pressure that produced the peak braking force is reestablished to establish a braking condition in which the wheel slip is substantially at the critical slip value for the existing road-tire interface condition. The braking pressure reestablished to produce the critical wheel slip value is automatically adjusted to compensate for changing brake system parameters and for other errors that may exist in the calculation of the braking force to provide for a system that self-adapts to those changes and errors.